DE102007012495B4 - METHOD FOR PRODUCING A DIFFUSION MEDIUM - Google Patents

Abstract

A method of making a diffusion medium (34, 36, 38, 40) comprising:
an aqueous dispersion comprising a powder resin, a binder material and a fiber material comprising carbon fibers, graphite fibers and combinations thereof is produced (110),
a layer of the dispersion is formed on a carrier (120),
Removing water from the layer to form a fibrous layer
the fiber layer is formed, and
the formed layer is carbonized or graphitized (140), completely burning away the binder material.

Description

FIELD OF THE INVENTION

The disclosure relates to a method for manufacturing fuel cell diffusion media.

BACKGROUND OF THE INVENTION

Fuel cells can be used as an energy source for electric vehicles and other applications. An exemplary fuel cell has a membrane electrode assembly (MEA) with catalytic electrodes and a proton exchange membrane (PEM) formed between the electrodes. Water is generated at the cathode electrode based on the electrochemical reactions between hydrogen and oxygen that take place in the MEA. The gas diffusion media play an important role in PEM fuel cells. Generally arranged between flow field channels introducing catalytic electrodes and reactant gases into the fuel cell, the gas diffusion media provide paths for reactant for diffusion to the electrode and paths for removing the product water, electronic conductivity and thermal conductivity as well as mechanical strength, which is necessary for the correct functioning of the fuel cell is.

During operation of the fuel cell, water is generated at the cathode on the basis of electrochemical reactions that affect hydrogen and oxygen and that take place in the MEA. Efficient operation of a fuel cell depends on the ability to provide effective water management in the system. For example, the diffusion media prevent the electrodes from being flooded (ie, filled with water and severely limiting O ₂ entry) by removing product water away from the catalyst layer while maintaining reactant gas flow from the gas flow channels of the bipolar plate to the catalyst layer.

Fuel cell stacks can contain a large number of fuel cells depending on the performance requirement of the application. For example, typical fuel cell stacks have up to 400 individual fuel cells and more. Because the fuel cells in the stacks work in series, weakness or poor performance in one cell can translate into poor performance in the entire stack. For this reason, it is desirable that each fuel cell in the stack operate with high efficiency.

Typical manufacturing steps for gas diffusion media include that a carbon fiber paper is made, the paper is impregnated with resin or a mixture of resin and fillers, the impregnated paper is molded and the resin impregnated carbon fiber paper is carbonized or graphitized. The paper making and impregnation steps are continuous, while the molding, carbonization, and graphitization steps can be either batch or continuous.

For example, the DE 103 42 199 A1 a method for producing a gas diffusion layer, in which a layer of a mixture of resin powder and carbon is deposited on a carbon-containing base body by an electrostatic powder spray process.

Furthermore describes the US 6,171,720 B1 a bipolar plate with an integrated diffusion layer. The bipolar plate is made from a fiberboard which is made from an aqueous slurry of a powdered phenolic resin binder and fibers and which, after manufacture, is impregnated with hydrocarbon to form a bipolar plate layer which is integrated into the fiberboard.

Starting from the US 6,171,720 B1 the invention is based on the object of achieving improvements in the manufacturing process which reduce costs, simplify the process and increase the performance of the media.

SUMMARY OF THE INVENTION

This object is achieved by a method according to claim 1 for producing a diffusion medium.

The invention provides a process for producing a gas diffusion medium in which (a) an aqueous dispersion comprising a fiber material, a resin powder and a binder material is produced, (b) the fiber and resin powder dispersion in a layer or mat on a support is formed, (c) water is removed from the layer to form a fibrous layer containing the resin powder, (d) the resin powder-containing fibrous layer is molded, and then (e) the molded layer is carbonized or graphitized, completely burning off the binder material becomes. This method avoids a step of impregnating the fiber mat with a resin.

In one embodiment, the carrier is a sieve with a desired mesh size. Fiber strands and resin powder particles that are larger than the holes in the mesh are retained to make the fiber layer, and smaller particles can also be retained in the fiber mat. In various embodiments, the sieve has a mesh size, which is smaller than the average particle size of the resin powder. In a particular embodiment, a resin powder with an average particle size between about 40 microns to about 100 microns is used.

In one embodiment of the process, the resin powder is a phenolic resin or comprises a phenolic resin.

In certain embodiments, the resin powder has an average particle size between about 40 microns and about 100 microns.

The invention further provides a diffusion medium made in accordance with the method of the invention. The diffusion medium has an increased gas permeability compared to a diffusion medium which has the same resin content, but was produced by methods according to the prior art. Without wishing to be bound by theory, it is believed that this is because the carbon or graphite fibers are spot welded together by the well-distributed resin particles that are present in carbon paper making compared to a solution impregnation process that relies on them that resin flows as it cures to hold the individual fibers together. The conventional impregnation step results in a thin resin-rich surface layer after resin impregnation and curing, which is believed to be converted to an amorphous carbon surface layer on the single fiber after carbonization / graphitization, which can affect the surface free energy and the gas diffusion media durability . This surface layer on the carbon fibers is avoided by the present invention.

The invention also provides a fuel cell that includes a membrane electrode assembly (MEA) that includes an anode, a cathode, and a proton exchange membrane (PEM) that is disposed between the anode and the cathode; an impermeable electrically conductive element on at least one of the cathode and anode sides of the MEA, which together with the respective cathode and / or anode defines a fluid distribution chamber between the impermeable electrically conductive element and the cathode and / or anode; and comprises a diffusion medium according to the invention arranged in one or both of the fluid distribution chambers. The diffusion medium preferably spans the fluid distribution chamber from the side of the impermeable, electrically conductive element to the electrode side.

The new carbon fiber paper manufacturing steps of the process reduce the cost of manufacturing the carbon fiber gas diffusion media and, consequently, the cost of the fuel cell made using the gas diffusion media of the invention. In addition, the diffusion medium made by the process of the invention has a more open pore structure and improved fuel cell performance.

In the description of the invention, "resin" includes both oligomeric and polymeric materials that are suitable for the particular application. “Powder” refers to ground, ground or granulated material. "About" has its usual meaning of "almost" or that there may be some slight inaccuracy in the value (roughly or reasonably close to the value). Numerical ranges are to be understood to disclose the endpoint values and each value in the range as well as all included ranges, which are formed by taking any two disclosed values as endpoints. The terms "powder resin" and "resin powder" are understood to mean resins that are in powder form.

Figure list

• 1 Figure 3 is a flow diagram of a process of the invention;
• 2a and 2 B are reproductions of SEM images of carbon fiber paper that are made using a conventional solution resin impregnation process ( 2a) and by the powder resin doping process of the invention ( 2 B) is manufactured;
• 3rd FIG. 4 is a schematic diagram of a fuel cell stack containing gas diffusion media; and
• 4th shows comparative current-voltage curves of treated diffusion media.

DETAILED DESCRIPTION

A gas diffusion medium is made with fibers and a powder resin. The powder resin is not solvated when integrated into the gas diffusion media precursor. The powder resin is integrated in its powder form and can be melted and at least partially cured prior to carbonization or graphitization to produce the gas diffusion medium from the gas diffusion medium precursor.

1 shows a process 100 of the invention for producing a gas diffusion medium. At step 110 becomes a dispersion of carbon and / or graphite fiber, powder resin and a Binder material formed. In certain embodiments, the dispersion comprises carbon fibers obtained by carbonizing polyacrylonitrile fibers, which are then chopped to desired lengths. In various embodiments, the carbon fibers have a diameter of about 5 to about 10 microns and a length of about 5 mm to about 10 mm. Graphite fibers can also be used.

Suitable examples of carbon or graphite fibers include, for example, carbonized or graphitized fibers made of polyacrylonitrile, copolymers made of acrylonitrile (in particular those which contain at least 90% by weight of the acrylonitrile monomer), cellulose, rayon, pitch and phenols.

Polyacrylonitrile (PAN) fibers are commonly made from the PAN polymer using a solvent spinning process. 12 to 14 micron diameter filaments are typically used to make the carbon or graphite fibers for fuel cell diffusion media. The PAN fiber can be stabilized in air at about 230 ° C, then carbonized and chopped to desired lengths. The carbonization can be carried out at about 1200 to 1350 ° C. in nitrogen in order to achieve a carbon fiber with at least 95% by weight of carbon. The carbonized cables can be chopped in lengths from about 3 to about 12 mm.

In one method, the chopped carbon and / or graphite fiber is then mixed with water, the powder resin and a binder material to form an aqueous dispersion. A binder material is preferably used to give the carbon fiber paper structural integrity that is made from the dispersion so that the paper can be processed without damage. Suitable examples of binder materials include, for example, polyvinyl alcohol (PVA), polyvinyl pyrrolidone (PVP) and other such water-dispersible polymers.

The powder resin may be a carbonaceous resin or powder. Suitable examples include, for example, phenolic resins, amino resins such as melamine resins and urea resins, epoxy resins, furan resins, acrylic polymers, phenoxy resins, epoxy-modified polyimide resins, unsaturated polyester resins, polyimide resins, polyurethanes, diallyl phthalate resins, maleimide resins, fluorocarbon resins and polymers or cyanate and other resins, such as teflon or other polymers, such as teflon or other polymers, such as teflon or other polymers, such as teflon or other polymers, such as teflon or other polymers, such as teflon or other resins, such as teflon . In certain embodiments, the powder resin is thermoset, either self-crosslinking, formulated with a crosslinker, or oxidatively curing. In a particular embodiment, the powder resin comprises a phenolic resin. Phenolic resins are preferred because of their high carbon yield (50% of starting weight) and because of their low cost, although other resins are possible. The resin chosen should be a solid at the temperature at which the dispersion is formed, insoluble in water, and used in powdered form. In certain embodiments, it is desirable to use a powder resin with an average particle size between about 40 microns and about 100 microns. When the dispersion is formed into a layer on a mesh screen, water and some powder resin particles smaller in size than the mesh size of the screen pass through the screen. Fibers and resin powder particles with a size larger than that of the mesh holes are retained to form the fiber layer. Particles smaller than the sieve mesh can also be trapped in the fiber layer. The sieve has a mesh size that is smaller than the average particle size of the resin powder. For example, a 300 mesh to 100 mesh screen can be selected.

The dispersion may contain about 40 to 80 weight percent resin powder, and preferably about 60 to about 75 weight percent resin powder based on the combined weights of the resin powder and the carbon fiber and / or graphite fiber.

If the dispersion contains a binder material, the dispersion can be formed into a layer or mat or paper using paper making techniques. The dispersion can be formed into carbon fiber paper filled with the powder resin using a wet process and conventional papermaking equipment. In such a process, the dispersion is fed into a "headbox" that drips the dispersion into a rotating, porous drum or wire screen with a vacuum dryer to remove the water. The web that is formed is drawn off the drum or wire and is usually completely dried in an oven or on hot large diameter rotary drums.

In conventional processes for making diffusion media, the paper making or mat making process must be followed by an impregnation step in which a resin in organic solvent is introduced into the carbon fiber matrix. The present invention does not require this step because the carbon fiber paper or the carbon fiber mat is formed with the powder resin already dispersed over it. This avoids significant time, costs, emissions and equipment compared to the conventional processes.

The carbon fiber paper or carbon fiber mat containing powder resin is then heated in an oxygen atmosphere to at least partially cure the resin (called B-staging). The degree of polymerization after B-staging is sufficient so that there is very little resin flow during a molding step that follows. Since the dispersion contains the powder resin, the drying step and the B-staging step can be combined in a single process step.

After the B-staging, the particle resin-prepared paper or the particle resin-prepared mat can be shaped continuously or punched into discrete layers up to approximately one square meter for the following shaping step. The particle resin-prepared paper or the particle resin-prepared mat is at step 130 molded into a desired thickness and shape and then at step 140 carbonized or graphitized to provide the gas diffusion medium. Generally, the binder material is completely burned off during the carbonization and graphitization step.

The B-staging step can also be integrated in the molding step. In this embodiment, the carbon fiber paper or carbon fiber mat containing powder resin is molded with a controlled ramp-like temperature profile to fully cure the resin.

At step 130 of in 1 In the process shown, the particle resin-prepared carbon fiber paper or the particle resin-prepared mat is compression-molded and completely hardened by heating under pressure. The optimal temperature and pressure depend, for example, on the particular powder resin chosen. For example, a typical phenolic resin can be fully cured at or above about 175 ° C under a pressure of about 400-550 kPa. For batch processes, the papers or mats (as previously cut) with silicone coated release papers are stacked between them at desired intervals to provide the desired layers in the molded product or products. The stack can be molded at a given pressure or thickness to achieve the desired thickness or density for the molded papers. After molding, post-curing ("C-staging") can be carried out at an elevated temperature (for example at or higher than 200 ° C) in air to ensure complete curing or crosslinking of the resin before the next carbonization step.

Continue with step 140 the shaped paper or mat is carbonized or graphitized. Graphitization occurs when the mat is heated above 2000 ° C in an oxygen-free environment, which causes changes in the physical fiber structure. Amorphous carbon is converted to crystalline lamellar graphite, which results in higher tensile modulus, increased electrical and thermal conductivity, and higher density and chemical resistance compared to amorphous carbon fiber. The resin section of the composite does not graphitize, but remains as amorphous carbon.

Carbonization and graphitization can be carried out continuously or by stacking layers in a horizontal or vertical chamber or batch furnace. The stacks are heated in an inert atmosphere (e.g. nitrogen or argon). It is preferred to carbonize the composite in the same furnace in a continuous cycle (temperature 1200-2000 ° C) and then graphitize (temperature> 2000 ° C).

The 2a and 2 B are reproductions of SEM images of carbon fiber paper that are made using a conventional solution resin impregnation process ( 2a) and by the powder resin preparation process of the invention ( 2 B) is made. As can be seen, the process of the invention provides a more open, homogeneous structure. It is also believed that the conventional resin solution impregnation process leaves a thin carbonized resin film layer on the surface of the single fiber. This amorphous surface layer is avoided in the process of the invention, and a substantial portion of the original carbon fiber surface is left exposed, while the prior art products made by solution impregnation of fiber paper provide the fibers fully or almost completely coated with amorphous carbon film . Exposing the carbon fiber surface improves the conductivity, both thermally and electrically, and also the surface energy compared to products according to the prior art coated with amorphous carbon film.

In various embodiments, the diffusion media of the invention are surface coated to increase water transport through the diffusion media. Non-limiting examples of diffusion media treatments include polytetrafluoroethylene (PTFE) treatment and a coating with a microporous layer (MPL) that contains a blend of carbon powders and fluoropolymers. In one embodiment, a microporous layer is formed by coating the diffusion medium with a paste which contains carbon black (for example acetylene black) and PTFE dispersion, and the diffusion medium is sintered with the paste.

The diffusion medium is used to manufacture a fuel cell. Fuel cell stacks contain a variety of fuel cells, the number of individual cells depending on the power and voltage requirements of the application. In automotive applications, include typical fuel cell stacks 50 or more individual fuel cells and can contain up to 400, 500 or even more. Performance requirements in various applications can also be met by providing a number of modules that include individual fuel cell stacks. The modules are designed to work in a row to provide adequate performance and are sized to fit within the available housing.

3rd Fig. 4 is an exploded view showing some details of the construction of a typical multi-cell stack, with only two cells shown for clarity. The bipolar fuel cell stack 2nd has a pair of membrane electrode assemblies (MEA) 4th and 6 separated from each other by an electrically conductive fuel distribution element 8th , hereinafter bipolar plate 8th , are separated. The MEAs 4th and 6 and the bipolar plate 8th are between stainless steel clamping plates or end plates 10th and 12th and end contact elements 14 and 16 stacked together. The end contact elements 14 and 16 as well as both working sides of the bipolar plate 8th contain a flow field from a variety of grooves or channels 18th , 20 , 22 or. 24th to distribute fuel and oxidant gases (ie hydrogen and oxygen) to the MEAs 4th and 6 . Non-conductive sealing elements 26 , 28 , 30th and 32 provide seals as well as electrical insulation between various components of the fuel cell stack. The gas diffusion media of the invention 34 , 36 , 38 and 40 press against the electrode sides of the MEAs 4th and 6 . The end contact elements 14 and 16 press on diffusion media 34 or. 40 while the bipolar plate 8th to a diffusion medium 36 on the anode side of the MEA 4th and a diffusion medium 38 on the cathode side of the MEA 6 presses. Oxygen is sent to the cathode side of the fuel cell stack from a storage tank 46 via suitable supply piping 42 delivered while hydrogen to the anode side of the fuel cell from a storage tank 48 through suitable supply piping 44 is delivered. Alternatively, ambient air can be supplied to the cathode side as an oxygen source (e.g. from a compressor or blower) and hydrogen can be supplied to the anode from a methanol or gasoline reformer. For both the hydrogen and oxygen sides of the MEAs 4th and 6 discharge tubing (not shown) is also provided. An additional piping 50 , 52 and 54 is provided to apply liquid coolant to the bipolar plate 8th and the end plates 14 and 16 to deliver. Suitable piping for the discharge of coolant from the coolant bipolar plate 8th and the end plate 14 and 16 is also provided, but not shown.

Individual fuel cells contain a proton exchange membrane, which is arranged between electrodes. The electrodes are an anode and a cathode for use in carrying out the entire production of water from fuel containing hydrogen and an oxidant gas containing oxygen. In various embodiments, the electrodes contain carbon support particles on which smaller catalyst particles (such as platinum) are arranged, as well as a polymer electrolyte that serves to bind and conduct protons in the electrode layers. Suitable electrodes are commercially available and the cathode and anode can be made from the same materials. The electrodes are in contact with a porous and electrically conductive material, such as carbon cloth or carbon paper, which serves as the diffusion medium.

4th shows test data from individual 50 cm ² fuel cells. In all cases, the diffusion media of the present invention and the reference diffusion media were used on the cathode side of the fuel cell where the water management requirements of the diffusion media are most stringent. A conventional diffusion medium (Toray TGP H-060) treated with 7% by weight polytetrafluoroethylene was used as the anode diffusion medium. The fuel cell performance test was carried out at an absolute pressure of 270 kilopascals (kPa), a cell temperature of 60 ° C with both the anode and cathode dew points at 60 ° C, an anode flow of hydrogen at twice the stoichiometric requirement based on the current draw and a cathode flow of air performed at twice the stoichiometric requirement, resulting in a relative humidity (RF) of about 307% at the gas outlet. This test compares the effectiveness of liquid water management capability for the tested diffusion medium used on the cathode side of the cell. The comparison curve C1 carries the current versus the cell voltage for Toray 060 Carbon fiber paper that has been treated with 7 wt .-% PTFE. The comparison curve C2 shows the result for Toray 060 Carbon fiber paper, but coated with a coating of a microporous layer (MPL) with about 1.1 mg / cm ² , which was positioned on the catalyst layer. The MPL coating was made by rod coating a paste on the carbon fiber paper. The paste contained 2.4 g of acetylene black, 1.33 grams of a 60% PTFE dispersion, 31.5 ml of isopropanol and 37 ml of deionized water. Then the carbon paper with the paste is sintered by heating at 380 ° C for 20 minutes. The invention curves E1 and E2 show a current versus cell voltage for a carbon fiber paper made according to the invention using Sigrafil C-30 carbon fibers and a Rutger-Plenco 12114 resin powder with a mesh size of 120 was produced. This resin-bonded carbon fiber paper contains about 66% by weight resin before carbonization. The weight ratio of carbon fiber to carbonized resin is about 1: 1 after carbonization. The example of E1 is treated with PTFE (7% by weight) and the example of E2 is treated with PTFE (7% by weight) with an additional MPL coating which has been positioned on the catalyst layer. The MPL coating was made as before with a load of 1.15 mg / cm ² solids on the substrate.

The curves of 4th show that Examples E1 and E2 of the diffusion media of the invention provide significantly improved water management efficiency over the Toray diffusion media under high humidity conditions. Additional tests were also performed under fairly dry operating conditions without a performance difference between the invention sample and the reference sample being observed.

Claims (9)

A method of making a diffusion medium (34, 36, 38, 40) comprising: an aqueous dispersion comprising a powder resin, a binder material and a fiber material comprising carbon fibers, graphite fibers and combinations thereof is produced (110), a layer of the dispersion is formed on a carrier (120), Removing water from the layer to form a fibrous layer the fiber layer is formed, and the formed layer is carbonized or graphitized (140), completely burning away the binder material. Procedure according to Claim 1 wherein the fiber layer is at least partially cured (130) prior to molding. Procedure according to Claim 2 , wherein the removal of water and the at least partial hardening are carried out together. Procedure according to Claim 1 wherein the powder resin comprises a phenolic resin. Procedure according to Claim 1 , wherein the powder resin has an average particle size between 40 microns and 100 microns. Procedure according to Claim 1 wherein the powder resin comprises a phenolic resin. Procedure according to Claim 1 , wherein the fiber layer has 40 to 80 wt .-% powder resin before carbonization. Procedure according to Claim 1 , the carrier being a sieve. Procedure according to Claim 8 , wherein the sieve has a mesh size which is smaller than the average particle size of the powder resin.

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